Two ionic liquids based on guanidinium cations and TFSA(-) anion were prepared, and their electrochemical stabilities were investigated. The cathodic limiting potentials of the two ILs were 0.7V versus Li/Li+ and their electrochemical windows were 4.2V. However, the lithium plating and striping on Ni electrode Could been Observed in the two IL electrolytes containing 0.3 mol kg(-1) of LiTFSA Without additive. And Li/LiCoO2 cells using the two IL electrolytes without additive showed good capacity and cycle property at the current tale of 0.2 C. (C) 2009 Elsevier B.V. All rights reserved.